This map shows areas on the globe where there are disturbances in the Earth's magnetic field. Here, red indicates a stronger magnetic tug, and blue a weaker one (white lines are the edges of tectonic plates). As you can see, disturbances are fairly regularly distributed, but that doesn't stop UFO enthusiasts from saying aliens have a hand in these magnetic fluctuations. Science, however, has a slightly different explanation for why certain objects (including submarines) create a shift in the magnetic field.

According to the Geological Survey of Finland, which created the map out of years of survey research:

This map is the first global compilation of the wealth of magnetic anomaly information derived from more than 50 years of aeromagnetic surveys over land areas, research vessel magnetometer traverses at sea, and observations from earth-orbiting satellites, supplemented by anomaly values derived from oceanic crustal ages. The objective is to provide an interpretive dimension to surface observations of the Earth's composition and geologic structure. Metamorphism, petrology, and redox state all have important effects on the magnetism of crustal materials.

The magnetic anomalies represented on this map originate primarily in igneous and metamorphic rocks, in the Earth's crust and possibly, uppermost mantle. Magnetic anomalies represent an estimate of the short-wavelenght (< 2600 km) fields associated with these parts of the Earth, after estimates of fields from other sources have been subtracted from the measured field magnitude. In most places the magnetic anomaly field is less than 1 per cent of the total magnetic field.